Municipal solid waste (MSW) handling and disposal has received substantial attention by regulatory agencies, as well as by interested environmental groups. For the purpose of this application, MSW is defined as the gross product which is collected and processed by municipalities and governments. MSW includes durable and non-durable goods, containers and packaging, food and yard wastes, and miscellaneous inorganic wastes from residential, commercial and industrial sources. Examples include newsprint, appliances, clothing, scrap food, containers and packaging, disposable diapers, plastics of all sorts including disposable tableware and foamed packaging materials, rubber and wood products, potting soil, yard trimmings and consumer electronics, as part of an open-ended list of disposable or throw-away products.
Disposing of waste in a landfill is the most traditional method of waste disposal, and it remains a common practice in most countries. Many local authorities, especially in urban areas have found it difficult to establish new landfills due to opposition from owners of adjacent land, since few people want a landfill in their local neighborhood. As a result, solid waste disposal in these areas has become more expensive as material must be transported for disposal.
The substantial amount of MSW may be reduced by combustion at municipal solid waste combustors (MWC), also known as waste-to-energy (WTE) plants. Due to the need to decrease the amount of material placed into landfills, along with improved operating equipment and procedures that minimize resulting pollutants, WTE plants are finding increased acceptance and popularity.
One by-product of WTE plant operation is ash. The ash generally represents about one-fourth of the mass of MSW prior to processing. Various techniques are known for the disposal of MWC ash. For example, electric plasma torches may be used to melt incinerator ash into inert glassy pebbles that are valuable in concrete production. MWC ash may also be chemically separated into lye and other useful chemicals. Alternatively, the ash may be incorporated in portland cement furnaces. However, these techniques all have severe disadvantages, such as requiring complex and costly machinery or chemical treatment.
The most common technique for disposing of MWC ash is to deposit it into landfills. However, the ash must be tested and treated as needed for safe landfill disposal to prevent contamination of underground aquifers. The potential dangers of hazardous contaminants found in the ash, and in particular, heavy metals such as lead, arsenic, chromium, copper and cadmium, have been well documented and are the subject of numerous regulatory controls to reduce or eliminate the dangers to people and to the surrounding environment.
Solid waste disposal is regulated by both federal and state agencies through laws and regulations. In particular, the Resource Conservation and Recovery Act (RCRA) and the regulatory agencies operating under the Act, as embodied in Title 40 of the Code of Federal Regulations and its state regulatory equivalents, have established maximum safe limits for certain contaminants in solid waste. In 1995, in response to the United States Supreme Court decision in City of Chicago v. Environmental Defense Fund (114 S. Ct. 1588 (1994) regarding the RCRA requirement to test ash from MWCs, the Environmental Protection Agency issued guidelines for testing ash samples utilizing the toxicity characteristic leachate procedure (TCLP), also referred to as EPA Testing Method 1311 and described in EPA publication SW846, to determine whether the ash exhibits the characteristic of a hazardous waste. Before leaving the WTE plant, the MWC waste ash must pass test criteria for either final disposal in a landfill or reuse.
For example, 40 C.F.R. §261.24, contains a list of contaminants and their associated threshold concentrations in an extract of solid waste, as set forth below in Table 1:
TABLE 1Maximum Concentration ofContaminants for the Toxicity CharacteristicRegulatoryLevelContaminant(mg/L)Arsenic5.0Barium100.0Benzene0.5Cadmium1.0Carbon tetrachloride0.5Chlordane.0.03Chlorobenzene100.0Chloroform.6.0Chromium.5.0o-Cresol200.0m-Cresol200.0p-Cresol200.0Cresol200.02,4-D10.01,4-Dichlorobenzene7.51,2-Dichloroethane0.51,1-Dichloroethylene0.72,4-Dinitrotoluene0.13Endrin0.02Heptachlor (and its epoxide).0.008Hexachlorobenzene0.13Hexachlorobutadiene0.5Hexachloroethane3.0Lead5.0Lindane0.4Mercury0.2Methoxychlor10.0Methyl ethyl ketone200.0Nitrobenzene2.0Pentrachlorophenol100.0Pyridine5.0Selenium1.0Silver5.0Tetrachloroethylene0.7Toxaphene0.5Trichloroethylene0.52,4,5-Trichlorophenol400.02,4,6-Trichlorophenol2.02,4,5-TP (Silvex)1.0Vinyl chloride0.2
If a contaminant equals or exceeds its threshold concentration when tested using the TCLP analysis as specified at 40 C.F.R. §260.11 (described in greater detail below), then the material is classified as being toxic and cannot be deposited into a non-hazardous waste landfill or ash monofill without first being treated. In addition, there exist various states such as California, Michigan and Vermont which require additional leaching tests on solid waste in order to classify the waste, and these states direct the heavy metal leaching wastes to hazardous waste landfills.
The TCLP test is designed to simulate the leaching of heavy metals that could potentially occur over time in a solid waste landfill when precipitation passes through the deposited ash. The TCLP procedures distinguish between ash with low alkalinity and ash with high alkalinity, and determination of the alkalinity of the material is the first step in the TCLP procedure so the appropriate extraction test fluid can be used. These test procedures add a known amount of acid to a small aliquot of the ash suspended in solution and measure the pH of the solution. For low alkalinity ash (test solution pH <5) a mild acetic acid/sodium hydroxide/water extraction fluid is used (fluid 1 having pH value of 4.93) for the TCLP test. For high alkalinity ash (defined to be a test solution having pH greater than 5) an acetic acid/water extraction fluid (fluid 2 having a pH of 2.88) is used. In an exemplary TCLP test, a single extraction of waste ash is tumbled in an appropriate acetic acid extract for 16 to 20 hours. The extract is filtered, digested and analyzed to establish the leachability of the elements of concern. If the determined value is equal to or greater than the limit for that element, the waste is categorized as hazardous and must be specially processed for disposal.
Various known methods may be used to treat hazardous ash, such as the addition of various chemicals to remove or otherwise nullify the hazardous contaminants. However, these treatment processes are relatively expensive and complex. Moreover, it is generally desirable to reduce the dependence on potentially hazardous additives. Accordingly, there is a current need for a method to handle the MWC waste ash with reduced chemical additives and other processing.
Referring now to FIG. 1 (PRIOR ART), a MWC 100 receives and combusts the MSW 110. The combustion produces ash content that is classified as either bottom ash (BA) 120 or fly ash (FA) 130. At the present time in the United States, typically all of the ash streams are combined, and this combined stream is referred to as combined ash.
The term bottom ash is commonly used to refer to the grate ash, siftings 121 and, in some cases, the boiler ash stream. Approximately 90 percent of the bottom ash stream consists of grate ash, which is the ash fraction that remains on the stoker or grate at the completion of the combustion cycle. It is similar in appearance to porous, grayish, silty sand with gravel, and contains small amounts of unburned organic material and chunks of metal. The grate ash stream consists primarily of glass, ceramics, ferrous and nonferrous metals, and minerals. As explained in greater detail below, the bottom ash 120 comprises approximately 75 to 80 percent of the total combined ash stream in known combined ash systems.
The Fly ash 130 refers to the ash collected in the air pollution control system, which includes the scrubber ash and precipitator or baghouse ash. Boiler ash, scrubber ash, and precipitator or baghouse ash consist of particulates that originate in the primary combustion zone area and are subsequently entrained in the combustion gas stream and carried into the boiler and air pollution control system. As the combustion gas passes through the boiler, scrubber, and precipitator or baghouse, the entrained particulates stick to the boiler tubes and walls (i.e., boiler fly ash 131) or are collected in the air pollution control equipment (i.e., Flue gas fly ash 132), which consists of the scrubber, electrostatic precipitator, or baghouse. Ash extracted from the combustion gas consists of very fine particles, with a significant fraction measuring less than 0.1 mm in diameter. The baghouse or precipitator ash comprises approximately 10 to 15 percent of the total combined ash stream.
Thus, the FA 130 comprises lighter particles which are carried off the burning grate by convection or turbulence, boiler fly ash 131, or form in the flue gas cleaning system, flue ash 132. Fly ash 130 is removed by electrostatic precipitators or collection bags in a fabric filter. Fly ash can also include the superheater or economizer ash which collects on internal parts of the boiler system which are blown down or removed from time to time and combined with the fly ash fraction.
The flue ash 132 frequently contains spent lime from an air pollution control system (APC) in which a lime reagent is sprayed into the flue gases to neutralize sulfur dioxide and hydrochloric and other acids. The hot flue gases evaporate the water portion, leaving a dry powder residue 132 which is removed in a fabric filter and may be combined with the boiler fly ash 131.
Referring now to FIG. 2 (PRIOR ART), a conventional ash processing method 200 is presented. In summary, during the conventional method of ash mixing and disposal 200, all of the FA and the BA from a plant are combined, and the resulting combined ash (CA) is subjected to possible treatment and periodic TCLP testing before it is disposed in a non-hazardous landfill. Specifically, after the incineration of the solid waste in step 210 and the collection of the bottom ash 120 from different pieces of equipment (e.g., grate and boiler)and the fly ash 130 from flue gas treatment equipment (e.g., scrubbers and fabric filters), respectively steps 220 and 230, it is common practice at MWC facilities to mix all the bottom ash 120 and fly ash 130 to form CA, step 240.
The CA exiting the MWC facility must meet the TCLP requirements of a non-hazardous material to be acceptable for disposal in a non-hazardous solid waste landfill or ash monofill. Thus, the CA is tested in step 250, typically using the TCLP guidelines as described above, and if found to be hazardous, the CA must be treated in step 260 using various known techniques prior to disposal of the treated CA in step 270. In general, one goal of processing the ash is to achieve a stabilized material with sufficient alkalinity (e.g., pH range of approximately 8-11 for the TCLP extracted material as described in greater detail below) to prevent any acidic attack from allowing the solubility of toxic metals in the leachate solutions.
It should be appreciated that the BA or FA may also be treated prior to combining the ashes in step 240. For example, the FA from an MWC air pollution control system (APC) is frequently treated using a calcium oxide/hydroxide lime slurry spray dryer absorber with a fabric filter. This treated FA will then contain excess alkalinity due to un-reacted calcium oxide/hydroxide, whereas the BA is typically a low alkalinity material. Due to the high alkalinity of the FA from the flue gas treatment with its relatively high calcium content, CA appears to be mostly alkaline, but often at the low end of the desired pH range and at times even lower. When all the BA is mixed with the FA, as is the usual practice in step 240, the resulting material has a diluted total alkalinity, which can result in unacceptably low pH values in testing step 250 that permit the solubility of toxic metals in the extract. Consequently, the standard practice of mixing all the BA with the FA in step 240 is counterproductive to the goal of achieving a stabilized end material with sufficient alkalinity to prevent any acidic attack from allowing the solubility of toxic metals in the leachate solutions.
To address the low alkalinity in the CA and to otherwise improve the alkaline characteristics of the CA, it is often necessary to add an alkaline reagent, such as additional lime or dolomitic lime, to the CA stream during treatment step 260. Also, the alkaline reagent is often added to CA in step 260 as precaution due to the non-homogeneous nature of MSW, which causes the CA during testing step 250 to occasionally exhibit alkaline characteristics using fluid determination testing and acidic characteristics using extraction testing.
This alkaline reagent treatment results in considerable capital and operating expenses, including the costs associated with special equipment, the amendment reagent and increased quantities requiring disposal (due to the increase in overall volume and weight of the ash). Accordingly, it is a goal of the present invention to provide an ash processing methodology that is less reliant on the addition of an alkaline reagent.
Moreover, the overall combined ash testing and treatment process is wasteful in that potentially useful materials from the incineration are not reused. Specifically, another problem with the known ash processing method 200 is that after the BA from the combustor grate and boiler is mixed with the FA from the flue gas treatment system, the BA becomes coated with reacted and un-reacted lime and, perhaps, activated carbon. These spent reagents and reaction products are sticky and adhere to the BA, making it difficult to recycle materials in the BA. It is often not practical to wash the spent reagents and reaction products off the BA since a large stream of waste water and slurry would be created that contains the FA and reagents. This waste stream would be very difficult to dispose of in a landfill. Therefore, there is a need for an ash management method that would not only reduce or eliminate the need for the addition of amendment reagents, but would also leave a large portion of the BA free from contamination for potential recycling (i.e., in asphalt or concrete).